Jack Ivey wrote:
> Bret Nordgren wrote:
> 
>> Another factor that you may want to consider is thermal variation.  At very
>> low frequencies, below 1Hz, the effects of micro-variations in the device
>> temperature can add additional "noise". 
(snip)
> 
> I've seen this effect with thermocouple amplifiers, where moving your hand
> near the circuit would move the air enough to create low-frequency noise.
> It can be almost eliminated by pressing the circuit board between pieces
> of foam rubber.

It also helps a lot to keep the internal temperature rise of the front
end opamp to a minimum.  Reducing the opamp supply voltage as much as
possible without degrading the performance of the amp helps keep the
chip cool and reduce the thermal effect of changes in air currents.

For this reason, if two amp choices have similar noise specs, but one
may be operated at lower supply voltage or draws less supply current,
its lower self heating may allow it to out perform its hotter
competition in the low frequency realm.

Thanks to both of you for reminding me of this subtlety. 

-- 
John Popelish
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